ASoC: bindings: fsl-asoc-card: Add compatible for tlv320aic31xx codec

Commit 8c9b9cfb

 ("ASoC: fsl-asoc-card: Support
fsl,imx-audio-tlv320aic31xx codec")' added support for tlv320aic31xx
codec to fsl-asoc-card, but missed the related device-tree compatible
string documentation. Fix this.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Link: https://lore.kernel.org/r/20220210134049.32576-1-ariel.dalessandro@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>